258 



ZOOLOGY. 



the preceding class, and is, by some authors, regarded as 

 the type of a separate class (Scaphopoda). The sexes of 



Fig. 181. Development of Dentallum. A, morula ; B, trochosphere ; C, annu- 

 lated larva ; D, larva with its rudimentary shell ; z, velum ; a, shell ; E, young much 

 farther advanced, the shell or body segmented ; d, rudimentary tentacles ; .;, sub- 

 oesophageal nerve-ganglia ; //", digestive canal, and liver (/') ; the foot protrudes 

 from the shell. All magnified. After Lacaze-Duthiers. 



Dentalium are distinct. The young is a trochosphere and 

 afterwards becomes segmented, and the univalve 

 shell then appears. (Fig. 182.) 



Order 2. Pteropoda. In these winged-snails 

 the head is slightly indicated and the eyes are 

 rudimentary ; while they are easily recognized by 

 the large wing-like appendages (epipodium), jne 

 on each side of the head. The shell is conical 

 or helix-like. The species are hermaphroditic. 

 Cavolina tridentata Lamarck and Styliola vitrea 

 Verrill (Fig. 183) are pelagic forms, occurring on 

 seas, and are occasionally taken with the 



SSu nSn e ey - tow - net off tne southern coast of New England. 

 After steams'. Limaciua arctica Fabr. is of the size of, and 

 looks like, a sweet pea, moving up and down in the water. 

 It is common from Labrador to the polar regions. 



